1. Field of the Invention
This invention relates to the extrusion coating of polyethylene terephthalate (PET) onto paperboard. Such structures of this type, generally, provide an enhanced film adhesion of the PET onto the paperboard, while reducing the coat weights of the PET.
2. Description of the Related Art
When paperboard is extrusion coated with a polymer, the film formed during the process must have adequate adhesion to the paperboard substrate in order to withstand subsequent converting and end use requirements. When extrusion coating the polymer, adhesion is the limiting factor as to the coat weight or thickness necessary to provide an acceptable product. If adhesion could be improved at lower weights, the reduction in material to manufacture an acceptable product would be of great value.
Coating the paperboard substrate prior to extrusion with a primer has been attempted. Exemplary of such prior art is U.S. Pat. No. 3,371,002 (""002) to N. G. Reddeman, entitled xe2x80x9cLow Temperature Polyolefin Extrusion Coating Process.xe2x80x9d While the ""002 patent describes priming of a paper substrate, drying the substrate in an oven, and corona treating of the extrudate for improving adhesion of the extrudate to the substrate, the drawbacks of this method are mainly threefold. One, extrusion lines do not have the necessary coating system in-line; therefore, a separate converting step is necessary, which is costly and creates more waste. Secondly, the additional cost of the primer material is not justified by the amount of coat weight reduction which may be achieved. Finally, end use conditions for PET coated paperboard require that the material be able to withstand temperature extremes from xe2x88x9240xc2x0 F. to 400xc2x0 F. without delaminating. Typical ethylene-vinyl acetate (EVA) primers cannot withstand these extremes.
It is also known for adhering PET to paper, to heat an uncoated paper surface to at least 285xc2x0 F. by means of flame or hot gas. Exemplary of such prior art are U.S. Pat. No. 3,904,104 (""104) to W. P. Kane, entitled xe2x80x9cPolyethylene Terephthalate/Paperboard Blank and Container Formed From Such Blank,xe2x80x9d U.S. Pat. No. 3,924,013 (""013) to W. P. Kane, entitled xe2x80x9cMethod of Cooking Food in a Polyethylene Terephthalate/Paperboard Laminated Container,xe2x80x9d U.S. Pat. No. 3,939,025 (""025) to W. P. Kane, entitled xe2x80x9cMethod of Making a Polyethylene Terephthalate Laminate,xe2x80x9d and U.S. Pat. No.3,967,998 (""998) to W. P. Kane, entitled xe2x80x9cPolyethylene Terephthalate/Paperboard Laminate and Method of Making It, Container Blank Formed From Such Laminate and Container Formed From Such Blank, and Cooking Method Using Such Container.xe2x80x9d While the ""104, ""013, ""025, and ""998 references teach the use of heating an uncoated paper surface, the paper must be in the pH range of 7-7.5 and the intrinsic viscosity of the PET must be in a range of 0.51-0.85. Also, it must be pointed out that no coat weights are given in these references. Therefore, a more advantageous coating method would be presented if the paper surface could be heated without flame or hot gas and employ reduced coat weights.
Finally, it is known to heat the extrudate. Exemplary of such prior art are U.S. Pat. No. 5,770,274 (""274) to A. Christel, entitled xe2x80x9cMethod for Producing Extrusion-Coated Laminates,xe2x80x9d and U.S. Pat. No. 5,676,791 (""791) to A. Christel, entitled xe2x80x9cDevice for Producing Extrusion-Coated Laminates.xe2x80x9d While the ""791 and ""274 references teach the heat treating of the extrudate, this is done in order to improve adhesion of the extrudate to the substrate. Conversely, a more advantageous system would be presented if the substrate were heated.
It is apparent from the above that there exists a need in the art for a system which extrusion coats polyethylene terephthalate onto paperboard, and which at least equals the film adhesion characteristics of the known coating systems, but which at the same time substantially reduces the amount of PET being coated onto the paperboard. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.
The invention provides a method for coating an uncoated paperboard web with polyethylene terephthalate. The uncoated paperboard web is heated by infrared radiation to a temperature in the range of about 190 F. to about 260 F. Polyethylene terephthalate is extruded directly onto the heated uncoated paperboard web at a coat weight in the range between about 16 lbs./3000 ft2 and about 20 lbs./3000 ft2. The coated paperboard web is then surface treated.
It is preferred that the paperboard web be heated to a temperature in the range of about 140xc2x0 F. to about 255xc2x0 F., with the more preferred range being from about 210xc2x0 F. to about 250xc2x0 F.
It is also preferred that the extrudate has a coat weight of less than 20 lbs. per 3000 sq. ft., with a further preferred range being less than about 18 lbs. per 3000 sq. ft., and the most preferred range being less than about 16 lbs. per 3000 sq. ft.
In certain preferred embodiments, the coated paperboard web is brought into contact with a chill roll.
The preferred coating system, according to this invention, offers the following advantages: ease of assembly and repair; good stability; excellent durability; excellent economy; reduced coat weight and excellent adhesion. In fact, in many of the preferred embodiments, these factors of durability, economy, reduced coat weight, and adhesion are optimized to the extent that is considerably higher than heretofore achieved in prior, known coating systems.